It's happening everywhere right now! Plants are chopping off their own organs, and they are piling up in yards all over town! How come no one is worried about this epidemic of leaf death??!! Well, it happens every year, so I'm pretty sure the plants are going to recover. Still, why on earth would plants get rid of their most important organs? That's what we'll address in today's post.
|Closeup of leaf abscission zone on sourwood.|
|Fresh leaf scar where the abscission zone dissolved and the leaf fell off.|
|Sourwood leaves and petioles (stems) about to undergo abscission.|
|The abscission zone is at the base of the leaf petiole where it attaches to the twig.|
The loss of body parts comes at a huge cost. Plants work all summer to catch enough sunlight to grow more leaves and get bigger, and leaf abscission every fall would seem to waste that energy. But as with the lizards that lose their tails, there are also benefits. Lizards' bodies escape to live another day and regrow another tail. Plants benefit from shedding leaves by not having to maintain those leaves during the winter. Leaves are tender tissues that would become disfigured and die when frozen. Try putting some lettuce leaves in the freezer over night and then take them out to thaw. You will notice they turn to mush when they return to room temperature. In order for plants' leaves to survive winter, they would have to be tough, like holly, magnolia or spruce leaves, which take much more energy to produce. Plants with leaves that survive freezing grow more slowly than ones that shed their leaves.
|Dogwood with remnants of chlorophyll along veins and lots of anthocycanins (red pigment).|
Plants have many ways to minimize the costs of losing their leaves. They move all available nutrients out of their leaves and down into their roots to save the food for the next growing season. Leaves fall near the plant that grew them and decompose, releasing their nutrients into the soil and further increasing the amount of nutrients recovered by the plant. In this way, deciduous plants grow their own mulch. Some plants, like walnut trees, even deposit compounds in their leaves that suppress the growth of competitor plants as the leaves decompose throughout the winter and spring.
|Rainbow of fall colors.|
|Leaf scar on a buckeye showing scars where the leaf veins were sealed off with suberin.|